1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus including the zoom lens, and more particularly, to a zoom lens suitable for use in an image pickup apparatus using a solid-state image pickup element such as a video camera, an electronic still camera, a broadcast camera, or a surveillance camera, or an image pickup apparatus such as a camera using a silver-halide film.
2. Description of the Related Art
In recent years, image pickup apparatuses have continued to increase in functionality and decreased in size. As an imaging optical system for use in such an image pickup apparatus, there is a demand for a zoom lens having compact size, high zoom ratio, and high resolving power.
To achieve the desired compact size and to ensure a high zoom ratio in the zoom lens, the number of lenses in the lens units of the zoom lens is generally reduced while increasing the refractive powers of the lens units. In such a zoom lens, however, it is difficult to correct aberrations.
U.S. Pat. No. 6,449,433 and Japanese Patent Laid-Open No. 2004-117826 disclose zoom lenses in which five lens units respectively having positive, negative, positive, negative, and positive refractive powers are arranged in order from an object side to an image side. The side of a lens where an object to be imaged is placed can be referred to as the object side or front side of the lens; and the side of the lens where the image is formed can be referred to as the image side or back side of the lens.
To obtain high optical performance in such a five-unit zoom lens while achieving a high zoom ratio and a compact lens system, it is important to set the appropriate refractive powers of the lens units and to establish the correct moving parameters (conditions) of the lens units for zooming.
In particular, to obtain a high optical performance over the entire zoom range while ensuring a high zoom ratio of about 12, it is important to properly set the moving condition of the first lens unit for zooming and the refractive powers (the reciprocals of focal lengths) of the first and fourth lens units.